Sheet handling apparatus



Nov. 28, 1967 M. H. SEMENHUK SHE ET HANDLING APPARATUS Filed April INVENTOR /7/cHA EL H. SEMENHUK ATTORNEY United States Patent r 3,355,639 SHEET HANDLING APPARATUS Michael H. Semenhuk, Lyndhurst, Ohio, assignor to Harris-Intertype Corporation, Cleveland, Ohio, a corporation of Delaware Filed Apr. 5, 1965, Ser. No. 445,608 3 Claims. (Cl. 317-262) ABSTRACT OF THE DISCLOSURE A. Voltage doubling circuit where sides of two condensers are connected to a common junction which is connected to one side of an AC. source while the other sides of the condensers are connected to the other side of the AC. source and have rectifiers therein so that the condensersare charged on alternate half-cycles. Members which are to have a high voltage thereacross are connected across the two condensers. The connection between the common junction and the AC. source includes an impedance which limitsthe current to a safe value on a short circuit between the conductive members and the connections which connect the conductive members across the condenser also includes a resistor.

The present invention relates to electrostatic sheet hold down apparatus and particularly to such apparatus for use in printing presses.

In printing presses, electrostatic sheet hold downs are used to cause a sheet to hug a rotating cylinder as the sheet is carried by the cylinder. The hold downs generally comprise an electrostatic device for establishing a field between the device and the cylinder and for producing charged particles which are propelled toward the cylinder to charge the sheet moving therewith and cause the latter to be electrically attracted to the cylinder.

One requirement of electrostatic sheet hold downs is that a high voltage be used to provide ionization and an electric field to propel the ions. The high voltage presents problems when accidental short circuits occur between the electrostatic device and the cylinder.

Accordingly, an important object of the present invention is to provide a new and improved electrostatic unit for establishing a high voltage electric field between an electrostatic device and a member moving with a sheet to cause the latter to be attracted to the member, which unit has a circuit arrangement such that it is inherently protected should a short circuit occur between the electrostatic device and the member.

Another object of the present invention is to provide an electrostatic unit of the type described where the voltage at the output terminals immediately reduces in the event of an arc-over.

A further object of the present invention is to provide a new and improved electrostatic hold down unit as in the preceding object which utilizes a voltage doubling circuit to provide the high voltage electric field.

Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiment made with reference to the accompanying drawing in which:

FIG. 1 shows an electrostatic device and a power supply therefor in accordance with the present invention; and

FIG. 2 shows the circuit of FIG. 1 when a short circuit occurs.

As illustrated in the drawing, a sheet is to be caused to hug a rotating cylinder 11 which has grippers thereon for gripping the leading edge of the sheet. The cylinder 11 is illustrated as the impression cylinder of a lithographic printing press and cooperates with a blanket cylinder 14 to form a printing nip for printing the sheet 10.

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An electrostatic hold down device 15 is disposed adjacent the cylinder and comprises an elongated conductive bar '16 extending parallel to the length of the cylinder. The conductive bar 16 has a plurality of ionizing needles 17 arranged in a row along the length of the member and pointing toward the impression cylinder. The impression cylinder is a metallic cylinder and a high voltage is applied to the conductive bar 16 to cause an ionization of the air adjacent the bar to provide electric charges in the form of ions which are propelled toward the cylinder by the high voltage field between the electrostatic bar and the cylinder. The electric charge collects on the sheet to charge the sheet and cause it to be electrically attracted to the cylinder.

In accordance with the present invention the high Volt age direct current field is provided by a power unit 20. The power unit 20 comprises a step-up transformer 21 having a secondary 22 and a primary 23 connected to a l20-volt 60-cycle alternating current source. The secondary coil 22 has terminals 24, 25 which are connected by a rectifying and voltage doubling circuit to power output terminals 26, 27, the output terminal 26 being connected to the electrostatic bar 16 and the output terminal 27 being connected to ground. The secondary terminal 24 of the transformer is connected to the output terminal 26 through a rectifying diode 28 poled to conduct current flowing toward the secondary terminal 24. The secondary terminal 24 is also connected to the output terminal 27 through a rectifying diode 30 which is poled to conduct current flowing from the transformer terminal 24, and a resistance 31.

The secondary terminal 25 of the transformer 21 is connected to the output terminal 26 through a capacitor 33 and through a capacitor 34 connected in series in the order named betweenthe transformer and the output terminal 26. A condenser 34 has one side connected to a junction 36a between the condensers 33, 34 and its other side to a junction 36b between the diode 30 and the resistance 31. Resistances 37, 38 are respectively connected across the condensers 34, 35.

During normal operation, when the transformer terminal 24 is positive with respect to the terminal 25, current will flow from the terminal 24 through the diode 30 to charge the condenser 35 and the condenser 33. On the alternate half cycle, current will flow from the terminal 25 to reverse the charge on the condenser 36 and to charge the condenser 34, the charging current flowing through the diode 28 to the terminal 24. The resistors 37, 38 are high resistance bleed resistors which also stabilize the voltage across the condensers. On the half cycle when condenser 34 is being charged, the diode 30 blocks current and prevents the discharging of the condenser 35 and its charging in the opposite direction by the voltage on the secondary of the transformer. Similarly, when the condenser 35 is being charged, the diode 28 prevents the condenser 34 from being charged in the opposite direction by the voltage of the secondary coil 22. The condensers 34, 35 are in a voltage doubling arrangement and the voltage appearing across the resistors 37, 38 will be the sum of the voltages to which the condensers 34, 35 are charged.

While the condensers 34, 35 are connected in a voltage doubling arrangement, the voltage to which each of the condensers 34, 35 is charged will not be the voltage across the secondary coil 22 since in any half cycle, the condenser 33 is also charged and the sum of the voltages on the condensers 33, 34 on one half-cycle and the voltages on condensers 33, 35 on the other half-cycle approximate the output voltage across the secondary coil.

If a short circuit appears across the output terminals 26, 27, as is shown in FIG. 2, the condenser 35 and the resistance 31 is effectively connected in parallel with the condenser 34 for one half-cycle and the condenser 34 and the resistance 31 are connected in parallel with the condensers 35- on the other half-cycle. With this circuit arrangement, both the condensers 34, 35 are charged on each half cycle and since the condensers are now connected in parallel with each other, the reactive impedance in series with the condenser 33- is considerably less than it was without the short circuit and the major portion of the voltage drop appearing across the secondary coil now appears across the condenser 33 and a much smaller portion appears across the condensers 34, 35. Consequently, the voltage at the output terminals 26, 27 drops since only a very small portion of the voltage across the secondary 22 appears across the parallel circuit including condensers 34, 35. Because of the parallel circuit arrangement, this is also true for short circuits which present appreciable impedance.

While a short circuit between the terminals 26, 27 effectively connects the condensers 34, 35 in parallel with each other to reduce the effective impedance in series with the secondary coil 22, the reactive impedance of the condenser 33 will act to limit the current in the circuit. Moreover, as pointed out above, the resistance 31 is always in the branch of the parallel circuit including the output terminals 26, 27 and is used in the preferred embodiment to develop a voltage for an indicating light 40 of the glow tube type. The resistance 31 will also, on both half-cycles, limit the current flow through the short circuit and will aid in reducing the voltage at the output terminals 26, 2 7 when a short occurs. This is particularly helpful when the short circuit is due to an arc-over since the dropping of the. voltage may extinguish the arc. The indicating light 40 is normally out but will light to indicate short-circuit conditions when the current through the resistance 31 reaches a predetermined magnitude.

It can now be seen that the present invention provides a new and improved apparatus for electrostatically holding a sheet to a rotating cylinder in which the problems caused by a short circuit across the field are minimized.

While a preferred embodiment of the present inven tion has been described in detail, it is hereby intended to cover all modifications, constructions and arrangements which fall within the ability of those skilled in the art and within the spirit and scope of the present invention.

Having described my invention, I claim:

1. An apparatus of the character described comprising first and second conductive members which are to have a high voltage therebetween, and means for applying a direct current potential between said members comprising a source of alternating current having a pair of AC. terminals, first and second condensers, first circuit means connecting one side of said first condenser to one of said A.C. terminals, second circuit means connecting one side of said second condenser and said one of said A.C. terminals, each of said circuit means comprising a rectifier with said rectifiers poled to conduct on different half-cycles of the voltage across said A.C. terminals, the other sides of said first and second condensers being connected to a common junction, a connection from the other of said A.C. terminals to said junction, and additional circuit means connecting said condensers in series across said first and second conductive members, said common connection having impedance means therein effectively limiting current to a safe value on a short-circuit between said members.

2. An apparatus as defined in claim 1 wherein said impedance comprises a capacitive means.

3. An apparatus as defined in claim 2 wherein said additional circuit means includes a resistor independent of said first and second circuit means connected between one of said condensers and one of said members,

References Cited UNITED STATES PATENTS 3,290,580 12/1966 Wolff 321-15 3,275,922 9/1966 Meyer et al 321-15 3,177,421 4/1965 Montgomery 32115 2,772,371 11/1956 Denton .Q 307-- 2,683,852 7/1954 Sampson 321-18 MILTON O. HIRSHFIELD, Primary Examiner.

J. A. SILVERMAN, Assistant Examiner. 

1. AN APPARATUS OF THE CHARACTER DESCRIBED COMPRISING FIRST AND SECOND CONDUCTIVE MEMBERS WHICH ARE TO HAVE A HIGH VOLTAGE THEREBETWEEN, AND MEANS FOR APPLYING A DIRECT CURRENT POTENTIAL BETWEEN SAID MEMBERS COMPRISING A SOURCE OF ALERNATING CURRENT HAVING A PAIR OF A.C. TERMINALS, FIRST AND SECOND CONDENSERS, FIRST CIRCUIT MEANS CONNECTING ONE SIDE OF SAID FIRST CONDENSER TO ONE OF SAID A.C. TERMINALS, SECOND CIRCUIT MEANS CONNECTING ONE SIDE OF SAID SECOND CONDENSER AND SAID ONE OF SAID A.C. TERMINALS, EACH OF SAID CIRCUIT MEANS COMPRISING A RECTIFIER WITH SAID RECTIFIERS POLED TO CONDUCT ONE DIFFERENT HALF-CYCLES OF THE VOLTAGE ACROSS SAID A.C. TERMINALS THE OTHER SIDES OF SAID FIRST AND SECOND CONDENSERS BEING CONNECTED TO A COMMON JUNCTION, A CONNECTION FROM THE OTHER OF SAID A.C. TERMINALS TO SAID JUNCTION, AND ADDITIONAL CIRCUIT MEANS CONNECTING SAID CONDENSER IN SERIES ACROSS SAID FIRST AND SECOND CONDUCTIVE MEMBERS, SAID COMMON CONNECTION HAVING IMPEDANCE MEANS THEREIN EFFECTIVELY LIMITING CURRENT TO A SAFE VALUE ON A SHORT-CIRCUIT BETWEEN SAID MEMBERS. 